1. Field of the Invention
The present invention relates to devices and a method for protecting living plants from freeze damage. More particularly, the invention involves plant covers or tree trunk guards for frost prevention in the class of plant husbandry.
2. Background of the Invention
Citrus trees and other fruit trees and wooded plants are susceptible to cold weather when young. Young citrus trees are particularly vulnerable at the trunk location called the bud union where the trunk of the tree has been grafted to the root stock. If the bud union becomes frozen, the tree is lost. Several common practices have developed over the years to protect these young plants from deadly freeze. One common method involves piling dirt around the trunk. This method has many disadvantages. The banks of dirt must be built and torn down each year and the accumulated dirt has deleterious effects on the plant. These effects include promotion of fungus disease and deterioration resulting from the hot moist soil steaming the tree bark.
Other methods of tree freeze protection include insulating wraps made of materials such as rock wool, fiberglass and polyurethane foam. Among other disadvantages with these items, the rock wool has been found to harbor rodents and the fiberglass has been found to sag excessively under heavy rain. Only the polyurethane foam has found any commercial success but, if untreated, it deteriorates from exposure to sunlight. It also has the disadvantage that it requires two people for installation. One person will wrap a sheet around the tree and hold it while another person will tie it on or strap it in place. The ties or straps are undone in the springtime after the threat of a freeze has passed.
More sophisticated methods of freeze protection are taught in the U.S. Pat. Nos. 4,137,667, ('667), 3,466,799 ('799), 4,341,039 ('039) and 4,400,909 ('909). Patent '667 discloses a variety of double walled plastic containers which are placed around or over young plants. The containers provide some thermal insulation to the plant by creating a dead air space. Also, they are designed to have the space between their double walls filled with water to provide a low rate heat source at 32.degree. Fahrenheit through the latent heat of fusion. As the water freezes it releases heat to its surrounding environment for an extended period of time and maintains the temperature of the air surrounding the plant at or near 32.degree. Fahrenheit.
The teachings of patent '667 are useful for small seasonal plants started in the late winter but are impractical for use with citrus trees and the like. The usefulness of the '667 patent derives from its ability to completely enclose the plants in a shroud of plastic and water. That is, the '667 patent contemplates a frost protection method which requires total enclosure of the plant.
U.S. patent '799 discloses another device for plant protection. This device too is designed to totally enclose the plant from the stem base at ground level to the top of the foliage and is constructed of a thermal insulating material. Since the invention provides no heat source inside the thermal barrier, the device relies solely upon retention of trapped heat for protection of the plant.
As with the '667 patent, the device of the '799 patent would be impractical for use with citrus trees and the like. It would be difficult to design a total enclosure suitable to the wide variety of shapes and sizes of such plants. Furthermore, the device disclosed in patent '799 shares two other faults with the '667 patent device when applied to citrus trees. First, because it lacks an independent heat source, it is not useful when the ambient temperature falls much below 32.degree. Fahrenheit or when the ambient temperature is below freezing for a long period of time. Second, both devices envision total enclosure or coverage which is impractical with citrus trees due to the costliness of such methods. Furthermore, it has been found sufficient to protect only the lower portion of a young tree from freezing since the upper branches and leaves will rejuvenate if the trunk and, in the case of citrus trees, the bud union survives.
U.S. patents '039 and '909 teach an improvement upon the preceding solutions in the case of citrus trees and the like. These patents disclose, respectively, an insulating device designed to be secured to the tree trunk and an improvement upon such device providing insecticide. The device consists of two mirror image pieces which, when held together by a large clip, provide a complete thermal enclosure surrounding the tree trunk extending from the ground upward an indefinite distance. In its preferred embodiment, the two pieces have an interior recess in which a pesticide container and a liquid container are held by a splint. Pesticide vapors emanate from the pesticide container and the liquid container is filled either with water or some other liquid useful to provide heat through the latent heat of fusion which is released when the liquid undergoes transformation from the liquid to the solid state.
The devices taught by patents '039 and '909 have several limitations. As with all other methods described above, the heat source provided internal to the insulating barrier is not controllable or sustained. Even when treated to prevent supercooling before freezing, the water packets of the '039 and '909 patents have a limited quantity of energy available for release through the heat of fusion. Furthermore, these devices are not well sealed at the top of the insulating barrier and allow the heat to escape easily. A prolonged cold wave or a cold wave falling several degrees below 32.degree. Fahrenheit would result in tree trunk temperatures and, in the case of citrus trees, bud union temperatures falling below the deadly level of approximately 26.degree. to 28.degree. Fahrenheit.
The devices of the '039 and '909 patents have additional problems. Because they comprise two pieces held together by a clip, they easily can be knocked off by a passing animal or a strong breeze. If the two halves are knocked out of close contact and alignment, much of the insulating value is lost and the device becomes virtually useless to prevent freezing. Also, because the design involves packets held within the insulating shell, it is not well suited to on-sight construction and assembly. The result is that this device cannot readily be adapted to tree trunks of varying height. Unless the tree trunk is the exact height of the pre-assembled device of patents '039 and '909, or a multiple of that height, some otherwise protectable portion of the tree trunk goes unprotected. The additional tree loss due to freezing increases the time necessary for the tree to rejunvenate and, in some cases, may result in resprouting at awkward locations on the trunk.